Recently, ozone has been used for a variety of purposes such as the oxidation of ozone for forming an oxide film used in a semiconductor device, the ozone cleaning of a silicon wafer used in a semiconductor device, and water supply and sewerage treatment, and the like. The oxidation of ozone of a silicon wafer requires an oxide film of high quality, and hence, there is a demand for high-purity and high-concentration ozonized gas.
Further, in the ozone cleaning, ozone water obtained by dissolving ozonized gas in pure water is used as a cleaning solution, an organic substance and noble metal on a silicon wafer are removed by using the ozone water together with dilute hydrofluoric acid aqueous solution or the like.
In a semiconductor production field, in order to realize a more highly integrated semiconductor, each manufacturer is performing research and development competitively. Therefore, for example, the following is stated: in the step of producing a non-volatile semiconductor memory device (non-volatile memory) in which a silicon oxide film is formed on a nitrogen film of a silicon wafer, a control gate electrode and a floating gate electrode are formed of a very thin oxide film with a thickness of about 2 nm, and as means for forming an oxide film capable of suppressing a leakage current between layers, an oxide film of good quality can be formed by a low-temperature oxidation chemical reaction caused by 20 vol % (440 g/Nm3) or more of ultra-high-purity ozonized gas containing no impurities other than oxygen and ozonized gas and the irradiation with UV-rays and plasma discharge, whereby an oxide film satisfying the above-mentioned thickness and the prescription of suppressing a leakage current can be realized (for example, see Patent Document 1). In the semiconductor production field, in order to realize high integration, the formation of an oxide film of better quality is becoming important. Therefore, the technology enabling continuously supply of 20 vol % (440 g/Nm3) or more of ultra-high-purity ozonized gas, i.e., high-concentration ozonized gas in a large amount economically and safely.
It is difficult to store ozonized gas in a state of the ozonized gas as it is due to the strong self-decomposition property thereof, and the ozonized gas is generally generated for use by an ozone generator in the vicinity of an ozone-using facility.
As an apparatus for producing ozonized gas having high-concentration of 20% or more of a conventional example, there are two methods: an ozone adsorption method of allowing an adsorbent to adsorb ozonized gas and to desorb the adsorbed ozone, and an ozone liquefying method of liquefying ozonized gas to vaporize liquefied ozone.
In the technology of the ozone adsorption system, there is a method including an adsorption step of allowing an adsorbent cooled by a refrigerating machine to saturation-adsorb ozonized gas generated by an ozone generator in an atmospheric state, a purification step of discharging, under a reduced pressure, an adsorption/desorption column containing the adsorbent to a partial pressure of supply ozonized gas, and an elimination step of supplying ozone of high concentration with a pressure difference by communicating the ozone to an ozone-using facility kept in a vacuum state, while keeping a cooled state and a reduced-pressure state in the adsorption/desorption column (for example, see Patent Document 2).
Further, in another technology of the ozone adsorption system, the following method has been proposed: an ozone generator and three adsorption/desorption columns filled with an adsorbent are provided in parallel with each other, a gas piping configuration for supplying the ozonized gas (about 10 vol % (220 g/Nm3 or less)) in parallel to the three adsorption/desorption columns, a refrigerating machine for cooling (to −60° C.) the adsorption columns, a heater for controlling the temperature of the adsorption/desorption columns, an open/close valve for changing an ozone passage, an ozone storage container for storing desorbed ozone, and a mass-flow controller for adjusting the flow rate of ozone to be supplied from the ozone storage container to an ozone-using facility are provided, and four steps of an ozone adsorption step, a stabilizing and boosting step, an ozone desorption step, and a cooling step are repeated in time series in each of the three adsorption/desorption columns provided in parallel with each other while being shifted by ⅓ cycle to generate ozone of a concentration of 28.4 vol % continuously from the three adsorption/desorption columns (for example, see Patent Document 3).
Further, in a conventional discharge-type ozone generator, several % of nitrogen is mixed in oxygen gas to stabilize the generation of ozone (for example, see Patent Document 4).
In the technology of the ozone liquefying method, a refrigerating machine is cooled to a temperature at which ozone is liquefied, ozonized gas is supplied to the refrigerating machine to allow the ozonized gas to be liquefied selectively in the refrigerating machine, and the liquefied ozone is vaporized in the refrigerating machine in the subsequent step, whereby the ozonized gas is concentrated highly.
In a conventional apparatus for producing ultra-high-concentration ozone gas using the technology of the ozone liquefying method, ozonized gas (oxygen 90%-ozone 10%) generated from an ozone generator of about 10 vol % (220 g/Nm3) or less is supplied to a chamber cooled to 80 K (−193° C.) to 100 K (−173° C.) to liquefy only the ozonized gas, a gas part in the chamber is evacuated in a discharging device part in the subsequent step, the ozonized gas liquefied in the further subsequent step is heated with a heater to a temperature around 128 K (−145° C.) to 133 K (−140° C.) to form the evacuated gas part into 100% ozonized gas of 50 Torr (53 hPa) to 70 Torr (100 hPa) corresponding to a vapor pressure of ozone, and the evaporated ozone is taken out (for example, see Patent Document 5).    Patent Document 1: JP 2005-347679 A    Patent Document 2: JP 2002-68712 A    Patent Document 3: JP 11-335102 A    Patent Document 4: JP 2001-180915 A    Patent Document 5: JP 2001-133141 A